This invention relates to an improvement in the art of making high pressure laminated materials utilizing saturating kraft paper. More particularly, the invention relates to release sheets coated with an aqueous polymeric composition which makes the sheets useful in the production of heat and pressure consolidated laminated materials. Papers which arc coated with these aqueous polymeric compositions exhibit excellent release properties which, when employed as release sheets, permit high pressure laminates to be easily separated.
High pressure laminates are laminated articles typically made of a plurality of core sheets, a decorative sheet and, where desired, an overlay sheet. The sheets are coated with lamination polymers which are typically thermosetting materials, such as melamine or phenolic resins.
The core sheets are made from resin saturable paper (also known as saturating kraft paper). The number of core sheets used depends on the application requirements (i.e. strength and environment) and the desired thickness of the laminate. Laminates normally have a thickness ranging from 0.030 to 0.090 inch. The core sheets are typically saturated with phenolic resins.
The decorative sheets are commonly made of very heavy paper comprised of cellulose fibers and may contain an opacifying pigment (such as titanium oxide). While decorative sheets are usually printed upon with designs to create a decorative pattern for the laminate, the sheets may be left plain for industrial purposes. The decorative sheets are typically saturated with melamine resins.
The overlay sheets are clear cellulose sheets which act as a protective layer over the decorative sheet. These sheets are usually saturated with melamine resins. The overlay sheets are used optionally depending on the need for protective surfaces.
In a typical process for preparing laminates, the sheets are saturated with the appropriate thermoset resins and then stacked. The stacked sheet assembly is subsequently placed in a press and consolidated under heat and pressure. During this operation the resin flows sufficiently to displace air between the sheets. Simultaneously the resin polymerizes into a rigid solid, thereby producing a monolithic structure as the finished laminate composite.
It is economically desirable to produce a number of laminate panels from one pressing operation by placing a plurality of sheet assemblies in the press at the same time. FIG. 1 shows the layering of two sheet assemblies which are divided by release sheets. Release sheets are placed between the core layers of the two sheet assemblies to facilitate the separation of the two laminate panels by preventing the respective core layers from fusing together.
Traditionally, a sheet of glassine which has been treated on one side with a release compound (such as silicone, chromium complexes, and the like) is employed as a release sheet. After curing in the press, this sheet separates from the assembly adjacent to its treated surface and remains adhered to the other assembly. However, major problems exist with the utilization of such treated glassine papers as release sheets, in that the release compound interferes with glue adhesionxe2x80x94thereby requiring that the laminate be deep sanded to completely remove the glassine paper from the surface of the laminate. Moreover, the required sanding often causes the release of chromium and other complexes, thereby raising environmental concerns.
Several attempts have been made to produce commercially effective high pressure laminate release sheets. In U.S. Pat. No. 3,050,434, Emily Jr. et al. discloses a kraft paper core sheet impregnated with phenolic resin and coated with an alginic acid salt (such as sodium alginate) for use as a release sheet. However, such alginates are relatively expensive. Furthermore, it was found the alginate salt was mostly absorbed by the paper, thereby creating release problems. In U.S. Pat. No. 3,215,579, Hagen teaches release sheets which are first sized with an aqueous solution of water soluble alkaline earth or alkaline earth metal salts and then coated with sodium alginate. However, this process was costly and seemed to present release and salt absorption problems. Ward, in U.S. Pat. No. 3,898,114, teaches release coatings comprising poly(vinyl alcohol) and methyl cellulose. In U.S. Pat. No. 4,243,461, Jaisle et al. teaches release sheets comprising grades of resin-free paper having a water absorption of at least about 200 seconds sized with a water soluble salt of an earth metal or alkaline earth metal. Jaisle et al. further teaches, in U.S. Pat. No. 4,263,073, the coating of these release sheets with a mixture of triglyceride, lecithin, or hydrolyzed lecithin and a salt of alginic acid. However, the use of these type papers are relatively expensive. Gray, in U.S. Pat. No. 4,327,121, teaches the use of electron beam radiation to polymerize certain acrylic functional materials as coatings for release sheets. In U.S. Pat. No. 4,510,199, Brooker teaches release sheets comprising a thermosetting resin impregnated fibrous core sheet coated with a mixture of wax and alginate salt. The use of release sheets coated with a mixture of a silicone-acrylate release component, an acrylate-containing cross-linking agent, and an acrylic ester monomer or oligomer are taught by Lu in U.S. Pat. No. 5,425,991.
Despite these, and other, efforts there still exists a need in the art for an economical coated release sheet which can be employed to produce high pressure laminates.
Therefore, an object of this invention is to disclose an improved method of producing high pressure laminated materials.
Another object of the present invention is to disclose coated paper for use as release sheets in the production of heat and pressure consolidated laminates.
Other objects, features, and advantages of the invention will be apparent from the details of the invention as more fully described and claimed.
These and other objects of the invention, as embodied and broadly described herein, are met by an improved method of laminating sheets of acrylic or polyester resins which employs as a release sheet cellulosic-based paper substrates which have been coated on at least one side with an aqueous polymer coating composition. These release coating compositions are produced by reacting a mixture containing water-dispersible stabilizing polymer, vinylic monomer, and, optionally, fatty acid, in an emulsion polymerization reaction. Additional components may be added to give the coating compositions desired characteristics. Paper coated with these compositions produce release sheets exhibiting excellent separation properties for use in the production of high pressure laminates. Moreover, the polymeric nature of the release coating composition should permit employment of these release sheets without the need for extensive sanding to remove the coating in order to make the laminate surface susceptible to adhesive bonding.